Disadvantages
The amount of energy stored per unit weight is generally lower than that of an electrochemical battery (3–5 W·h/kg for a standard ultracapacitor, although 85 W.h/kg has been achieved in the lab[4] as of 2010 compared to 30–40 W·h/kg for a lead acid battery), 100-250 W·h/kg for a lithium-ion battery and about 1/1,000th the volumetric energy density of gasoline.
Has the highest dielectric absorption of any type of capacitor.
High self-discharge – the rate is considerably higher than that of an electrochemical battery.
Low maximum voltage – series connections are needed to obtain higher voltages, and voltage balancing may be required.
Unlike practical batteries, the voltage across any capacitor, including EDLCs, drops significantly as it discharges. Effective storage and recovery of energy requires complex electronic control and switching equipment, with consequent energy loss. A detailed paper on a multi-voltage 5.3 W EDLC power supply for medical equipment discusses design principles in detail. It uses a total of 55 F of capacitance, charges in about 150 seconds, and runs for about 60 seconds. The circuit uses switch-mode voltage regulators followed by linear regulators for clean and stable power, reducing efficiency to about 70%. The authors discuss the types of switching regulator available, buck, boost, and buck-boost, and conclude that for the widely varying voltage across an EDLC buck-boost is best, boost second-best, and buck unsuitable[12].
Very low internal resistance allows extremely rapid discharge when shorted, resulting in a spark hazard similar to any other capacitor of similar voltage and capacitance (generally much higher than electrochemical cells).
